West Nile deaths were mounting quickly this summer as mosquitoes spread the virus across the country. The situation was especially dire in Texas, where some authorities resorted to aerial spraying for the first time in decades to curb what became one of the worst such outbreaks in U.S. history.

Nationally, more than 240 people died from the mosquito-borne illness - about a third of them in Texas.

Now with the mosquito population decimated by the cooler weather, health experts have turned their attention to learning lessons from the latest round of deadly cases. Federal health authorities are collecting data and examining potential factors, while Dallas County - the epicenter of the outbreak - has begun year-round mosquito surveillance and testing.

What remains unclear is whether experts will be able to shed light on what caused the outbreak, why parts of Texas were so severely affected and if they can forecast the next major surge in the illness.

"I don't think that we're ever going to totally be able to sort it out," said Dr. Lyle Petersen of the Centers for Disease Control and Prevention. "For one reason, the ecology in the United States is extremely varied, so a factor that may affect an outbreak in Colorado may be different than a factor that causes an outbreak in Louisiana. The conditions in an urban area may be different than a suburban area."

West Nile virus is believed to have first appeared in the U.S. in 1999 in the New York City area and then gradually spread across the country. Mosquitoes get the virus from feeding on infected birds and then spread it to people they bite. Most people infected show no symptoms, but the most severe form of the disease, called neuroinvasive, can cause a coma, convulsions, muscle weakness, paralysis and death.

The Texas Department of State Health Services reported more than 835 neuroinvasive cases this year and 86 deaths, led by Dallas County's 18 fatalities. The national death toll this year approached historic numbers from 2002, when 284 people died from the disease.

Petersen said the CDC is trying to determine what caused the latest outbreak by looking at factors such as heat, rainfall and the number of migrating birds that transmit the virus to mosquitoes. The agency is also researching the genetics of the virus to see if it may have mutated, but that doesn't seem to have happened.

Petersen added that while a warm spring with ample rainfall in North Texas was likely a factor, it's not known exactly what caused so many cases in the area.

"Probably, there was just the right combination of warmer weather and enough rainfall and probably a good dose of bad luck as well," Petersen said. "These outbreaks are subject to a fair amount of random variation."

The situation became so severe in North Texas that the state paid for aerial spraying in areas of Dallas County and nearby Denton County. Dallas County had not conducted such an operation since 1966 when encephalitis was blamed for more than a dozen deaths.

The county began spraying insecticides from trucks with the first positive mosquito test in June and then added aerial spraying in August when it became apparent there were more areas with infected mosquitoes than they could cover with spray trucks, Dallas County Judge Clay Jenkins said.

"If you look at this outbreak, it came upon us very suddenly," Jenkins said. "We might have the same situation in the future where by the time people know there is a risk out there, dozens of people have already been infected."

Dr. Don Read knows firsthand how serious West Nile virus can be and has made a mission of spreading the word about taking precautions. The 70-year-old Dallas colon and rectal surgeon contracted the neuroinvasive form in 2005 and still wears leg braces from the ordeal.

"I tell people it's something you don't want to have," said Read, who formed a support group of survivors.

While there's no way to tell how bad West Nile will be - or which area will experience the worst outbreak - Dallas County has already made changes in anticipation of whatever warmer weather next year brings.

The county has started doing year-round mosquito surveillance and has added two microbiologists to handle the year-round testing, said Zachary Thompson, director of Dallas County Health and Human Services. Jenkins added that they can now get results from mosquito testing in one day instead of seven.

Thompson said they will also start their public information campaign earlier next year, advising residents to apply insect repellent, dress in long sleeves and long pants when outside, stay indoors from dusk to dawn and drain standing water on their property.

Thompson said all options will be considered to fight West Nile this coming year, including ground and aerial spraying. But he added that if residents follow their advice and targeted spraying is done, they may not have to do aerial spraying.

Jenkins notes, "Personal responsibility is important because we can't possible kill every West Nile mosquito."